Add cost functions example interpretation column

docs/src/modeler_guide/cost_functions.md

@@ -5,13 +5,13 @@ PowerSystems.jl provides an extensive type hierarchy to explicitly express relat
 To represent a cost for a particular [`Component`](@ref), the modeler first chooses one of the variable cost representations in the table below. Then, they wrap this [`ProductionVariableCostCurve`](@ref) in either a [`CostCurve`](@ref) to indicate a cost in currency or in a [`FuelCurve`](@ref) to indicate a cost per unit of fuel plus a fuel cost. Finally, the user creates a domain-specific [`OperationalCost`](@ref) that contains this variable cost as well as other costs that may exist in that domain, such as a fixed cost that is always incurred when the unit is on. For instance, we may have `RenewableGenerationCost(CostCurve(LinearCurve(22.0)))` to represent the cost of a renewable unit that produces at \$22/MWh, or `ThermalGenerationCost(; variable = CostCurve(LinearCurve(22.0)), fixed = 1.0, start_up = 2.0, shut_down = 3.0)` to represent the cost of a thermal unit that produces at \$22/MWh plus a fixed cost of \$1.0/hr, a start-up cost of \$2.0, and a shut-down cost of \$3.0. Below, we give the options for `ProductionVariableCostCurve`s. Information on what domain-specific cost must be provided for a given component type can be found in that component type's documentation.
 
 ## Variable Cost Representations
-For more details, see the documentation page for each type.
+For more details, see the documentation page for each type. "Example interpretation" assumes the cost is wrapped in a `CostCurve` with natural units. Note that all three `Piecewise` options here fundamentally represent the same curve.
 
-| Type alias | Description | Constructor parameters | Example |
-| :--- | :--- | :--- | :--- |
-| `LinearCurve` | Linear input-output curve with zero no-load cost (constant average rate) | Average/marginal rate | `LinearCurve(3.0)` |
-| `LinearCurve` | Linear input-output curve with nonzero no-load cost (constant marginal rate) | Marginal rate, cost at zero production | `LinearCurve(3.0, 5.0)` |
-| `QuadraticCurve` | Quadratic input-output curve, may have nonzero no-load cost | Quadratic, proportional, and constant terms of input-output curve | `QuadraticCurve(1.0, 1.0, 18.0)` |
-| `PiecewisePointCurve` | Piecewise linear curve specified by cost values at production points | Vector of (production, cost) pairs | `PiecewisePointCurve([(1.0, 20.0), (2.0, 24.0), (3.0, 30.0)])` |
-| `PiecewiseIncrementalCurve` | Piecewise linear curve specified by marginal rates (slopes) between production points, may have nonzero initial value | Cost at minimum production point, vector of $n$ production points, vector of $n-1$ marginal rates/slopes of the curve segments between the points | `PiecewiseIncrementalCurve(20., [1.0, 2.0, 3.0], [4.0, 6.0])` |
-| `PiecewiseAverageCurve` | Piecewise linear curve specified by average rates between production points, may have nonzero initial value | Cost at minimum production point, vector of $n$ production points, vector of average rates at the $n-1$ latter points | `PiecewiseAverageCurve(20., [1.0, 2.0, 3.0], [12.0, 10.0])` |
+| Type alias | Description | Constructor parameters | Example | Example interpretation |
+| :--- | :--- | :--- | :--- | :--- |
+| `LinearCurve` | Linear input-output curve with zero no-load cost (constant average rate) | Average/marginal rate | `LinearCurve(3.0)` | \$3/MWh |
+| `LinearCurve` | Linear input-output curve with nonzero no-load cost (constant marginal rate) | Marginal rate, cost at zero production | `LinearCurve(3.0, 5.0)` | \$3/MWh + \$5/hr
+| `QuadraticCurve` | Quadratic input-output curve, may have nonzero no-load cost | Quadratic, proportional, and constant terms of input-output curve | `QuadraticCurve(1.0, 1.0, 18.0)` | $C(P) = 1 P^2 + 1 P + 18$ where $C$ is \$/hr, $P$ is MW
+| `PiecewisePointCurve` | Piecewise linear curve specified by cost values at production points | Vector of (production, cost) pairs | `PiecewisePointCurve([(1.0, 20.0), (2.0, 24.0), (3.0, 30.0)])` | \$20/hr @ 1 MW, \$24/hr @ 2 MW, \$30/hr @ 3 MW, linear  \$/hr interpolation between these points
+| `PiecewiseIncrementalCurve` | Piecewise linear curve specified by marginal rates (slopes) between production points, may have nonzero initial value | Cost at minimum production point, vector of $n$ production points, vector of $n-1$ marginal rates/slopes of the curve segments between the points | `PiecewiseIncrementalCurve(20., [1.0, 2.0, 3.0], [4.0, 6.0])` | \$20/hr @ 1 MW plus additional \$4/MWh from 1 MW to 2 MW plus additional \$6/MWh from 2 MW to 3 MW
+| `PiecewiseAverageCurve` | Piecewise linear curve specified by average rates between production points, may have nonzero initial value | Cost at minimum production point, vector of $n$ production points, vector of average rates at the $n-1$ latter points | `PiecewiseAverageCurve(20., [1.0, 2.0, 3.0], [12.0, 10.0])` | \$20/hr @ 1 MW, \$12/MWh @ 2 MW, \$10/MWh @ 3 MW, linear  \$/hr interpolation between these points